Sometimes when doing nixos-rebuild switch --flake I need to install 20GB+

I setup my config take from unstable and install ‘some’ stable pacakges, but I don’t need all the unstable pacakges, I only need ‘some’ unstable pacakges.
If I switch my config the other way around so that almost all pacakges come from stable and only ‘some’ from unstable my issue is solved? Got it from here : Use unstable version for some packages